This invention relates to improving the stability of slurries of bulking kaolin clay pigments when the slurries are exposed to elevated temperatures, for example when the slurries are prepared into coating colors in a steam jet cooker or shipped and stored under high ambient temperatures.
Finely divided refined kaolin clay is widely used as a pigment to provide a glossy, white opaque surface finish on printing paper. The clay is applied as an aqueous "coating color" which comprises a clay pigment, a dispersing agent for the clay, a suitable adhesive such as a polymer latex, starch, or mixtures thereof and other minor additives. Present-day coatings are applied at high machine speeds which necessitate the use of high solids coating colors. The formulation of coating colors at high solids requires the initial provision of fluid high solids clay-water suspensions or "slips". These suspensions are subsequently mixed with adhesive dispersions or suspensions to prepare the coating colors. High solids clay-water suspensions of hydrous (uncalcined) clays generally contain in excess of 65% clay solids (65 parts by weight dry clay to 35 parts by weight water). Typically, solids are about 70%. A dispersing (deflocculating) agent, conventionally a sodium condensed phosphate salt, or sodium polyacrylate, is present in the high solids suspensions in order to impart fluidity, since the systems do not flow in the absence of the dispersing agent.
Kaolin clay pigments must meet certain requirements with regard to rheological properties and to the properties of the coated sheet material. The visosity of the high solids suspension of the clay coating pigment must be sufficiently low to permit mixing and pumping. After the adhesive is incorporated, the resulting coating color must also have suitable viscosity for handling and application to the paper sheet. In addition, it is highly desirable to obtain a coated calendered sheet which has good opacity, gloss, brightness and printability.
It is the conventional practice in the art to improve the opacifying or hiding power of coating colors by blending the clay pigments with more costly pigments having greater opacifying power, such as TiO.sub.2. The industry has long sought a kaolin clay pigment which imparts improved opacifying power to coated paper without sacrificing gloss and printability and which can preferably be used in the absence of other more expensive pigments.
High bulking clay pigments offer the opportunity of maintaining or improving the opacity, gloss and printability of coated paper incorporated at lower coating weights, thereby reducing the pigment cost for coating colors. Bulking pigments are those which provide coatings having high opacification at a low cost weight. Generally, bulking is achieved by introducing voids in a pigment structure which contribute to increase light scatter. Controlled calcination of kaolin clays results in one type of bulking clay pigment. U.S. Pat. Nos. 4,075,030; 4,076,548 and 4,078,941 teach increasing the opacifying power of hydrous kaolin clays by "selectively flocculating " ultrafine clay particles with a low molecular weight polyamine flocculating agent (e.g. ethylene diamine or hexamethylene diamine) or with long carbon chain amines or certain quaternary ammonium salts (e.g., "ditallowdimethyl" ammonium chloride) in the presence of a mineral acid flocculating agent, e.g., sulfuric acid, and optionally with the added presence of citric acid or mica or both. The selective flocculating treatment allegedly incorporates voids in the clay to form a low density, high bulking pigment which when used as a coating color pigment improves the opacity of paper coated therewith. These patents do not disclose the use of polymers not do they contain information regarding the ability to disperse the bulked clay to prepare clay-slurries having acceptable rheological characteristics for commercial use.
We are aware of the fact that efforts to exploit bulking pigments to the paper industry have been thwarted among other things by the poor rheology of the pigments. Generally, paper makers seek to use clay coating pigments capable of forming high solids clay-water slurries which have a low shear viscosity below 1000 cp, preferably below 500 cp when measured by the Brookfield viscometer at 20 rpm. High shear viscosity for these slurries should be such that they are no more viscous than a slurry having a Hercules endpoint viscosity of 500 rpm, preferably 800 rpm, using the "A" bob at 16.times.10.sup.5 dyne-cm. Those skilled in the art are aware that when using the Hercules viscometer and measuring endpoints of 1100 rpm or higher, endpoint viscosities are reported in units of dyne-cm at 1100 rpm; apparent viscosity increases as the value for dyne-cm increases. It is conventional to use the abbreviated term "dyne". Thus, a "2 dyne" clay slurry is less viscous than a "9 dyne clay" slurry. As used hereinafter the expressions 500 rpm or higher, or 800 rpm or higher, are intended to include lower viscosities such that endpoint measurements are at 1100 rpm and the values are reported as dynes.
For reasons of economy, the manufacture of refined kaolin pigments usually necessitates sizing and purifying crude clay while the clay is in the form of a fluid deflocculated aqueous slurry, bleaching the clay while in a flocculated state, filtering the flocculated clay to remove liquid water and then dispersing the flocculated clay to form a high solids slurry that is sold as such or is dried, usually in a spray dryer, to provide a dry refined pigment capable of being mixed with water to form a dispersed fluid suspension. The latter form of clay is frequently referred to a "predispersed" grade of clay even though the clay is dry and is not present in dispersed state until it is mixed with water. Another problem encountered in the manufacture of bulking pigments from clay is to produce a bulked structure that is sufficiently durable to survive during various stages of production and end-use but is also capable of being dispersed to form high solids clay-water slurries having acceptable rheology. When the general wet processing scheme described above is employed to make bulked structures by adding a bulking agent before filtration, the bulked structure must still be present in the filter cake containing the bulked assemblages when the filter cake is "made down" into a fluid slurry. The expressions "make down" and "made down" are conventional in the industry and refer to the preparation of dispersed pigment-water slurries. In some cases, it may be necessary to apply mechanical work to the filter cake to reduce the low shear viscosity to usable values. The bulked structure must be sufficiently tenacious to survive the mechanical forces during such treatment. Bulking pigments must also be sufficiently stable under the influence of shear to maintain the bulked structure under the high shear rates encountered in pumping high solids clay water slurries. Moreover, a bulked structure must be capable of being retained when the deflocculated clay water slurry is formed into a coating color using standard makedown equipment. Also, the bulked structure must survive during the coating application and subsequent calendering. The fragility of the bulked structures obtained by chemical treatments of hydrous clays has limited their commercial use. Commercial bulking clays heretofore used by the paper industry are prepared by calcining fine particle size hydrous clays. In such case, calcination "sets" the bulked structure such that it is sufficiently durable to survive during manufacturing handling and use. Generally, a criterion for durability of a bulked structure is the retention of improved opacification (light scattering).
The present invention makes use of water-soluble cationic polyelectrolytes to produce bulked hydrous clay pigments having a unique combination of desirable properties. These cationic polyelectrolytes are high charge density materials and have the ability to flocculate clay-water slurries Cationic polyelectrolyte flocculants have been used in the past to clarify various suspensions such as river waters containing suspended fine solids, municipal waste and sewage sludge. The efficiency of such flocculants is frequently evaluated by measuring the ability of the flocculant to clarify clay suspensions. It is also known that various polymers including certain cationic polyelectrolytes will increase the rate at which suspensions of clay filter. However, the use of polymeric filter aids to increase filtration may adversely affect the rheology of kaolin clay intended for use as high performance pigments in the paper industry. Furthermore, filter cake solids are usually decreased when polymers are used as filter aids. As a result drying costs are increased. This may reduce the economic benefit of increased filtration rates. To the best of our knowledge, the quality segment of the clay industry devoted to producing high performance pigments and fillers does not utilize polymeric filter aids to produce clay pigments.